1. Field of the Invention
The present invention concerns an electrode for measuring the activity of calcium ions in a solution. The invention also relates to a membrane and a composition for use in such electrode.
2. Prior Art
Calcium electrodes, that is, electrodes used for measuring the concentration or activity of calcium ions in a solution, are known in various embodiments, one of which is described in British Pat. Specification No. 1,107,108. Common to all known calcium electrodes is the fact that they contain an organic ion exchanger, either in solution and mechanically retained within a membrane, or incorporated in a polymer matrix of, e.g., polyvinyl chloride.
Several organic ion exchangers useful in calcium electrodes have been described in the literature: thus, Israelian Pat. No. 26,233 describes the use of thionyltrifluoroacetone + tributylphosphate of the formula EQU (CH.sub.3 --CH.sub.2 --CH.sub.2 --CH.sub.2 O).sub.3 P=O
British Pat. Specification No. 1,107,108 describes the use of di(2-ethylhexyl)phosphoric acid of the formula ##EQU1## and U.S. Pat. No. 3,445,365 describes the use of di(n-decyl)phosphoric acid of the formula ##EQU2##
As mentioned above, the ion exchanger -- hereinafter called the electroactive material -- may be in solution when employed in electrodes. Moreover, the electroactive material is present as wholly or partly converted into its calcium salt. The solvents used are organic solvents and can be of various types. Thus, U.S. Pat. No. 3,429,785 mentions as solvents n-decanol, n-dodecylalcohol, n-decylalcohol and di(n-octyl)phenylphosphonate of the formula ##SPC1##
Di(n-octyl)phenylphosphonate belongs to the class of solvents or carriers often called mediators which, through synergetic extraction, have a favourable influence on the quality of the electrodes, resulting inter alia in improved selectivity and a better detection limit.
The above-mentioned electroactive materials used in known calcium electrodes have several disadvantages, viz., low seletivity with regard to the calcium ions over hydrogen ions, sodium ions, potassium ions, etc.